The `SettingsBuilder` now accepts a PSR-16 cache interface with which to
store and query settings before attempting to load from each source. By
default, no cache is used, but any object that implements the
`Psr\SimpleCache\CacheInterface` may be provided to the constructor.
An explicit dependency on "psr/simple-cache" has been added to
`composer.json`. Note that this dependency already existed in vendor
albeit it as a transitive one.
An APCu based `SharedMemoryCache` adapter is provided as a canonical
PSR-16 compliant interface for production use.
Sources are now queued by the `SettingsBuilder` when calling `load()`.
If a cache interface has been provided, and the source is considered
cacheable (implements `CacheableSource`), then it is wrapped as a
`CachedSource` which will query the cache first before loading from the
wrapped source.
Cache stampedes are mitigated using probabilistic early expiry. The
implementation for this was partially based on symfony/cache-contract
source code but also from the Wikipedia article and paper referenced
therein.
See https://en.wikipedia.org/wiki/Cache_stampede#Probabilistic_early_expiration
Bug: T294748
Change-Id: I52ab3899731546876ee58265bd4a1927886746dc
A `SettingsSource` is meant to represent any kind of local or remote
store from which settings can be read, be this a local file, remote URL,
database, etc. It is concerned with reading in (and possibly decoding)
settings data, and computing a consistent hash key that may be used in
caching.
A `SettingsFormat` is meant to detect supported file types and/or decode
source contents into settings arrays. As of now, JSON is the only
supported format but others may be implemented.
`FileSource` is the first source implementation, with its default format
being JSON, meant to read settings from local JSON files.
`ArraySource` is mostly useful for testing using array literals.
Refactored `SettingsBuilder` methods to use the new source abstractions.
Bug: T295499
Change-Id: If7869609c4ad1ccd0894d5ba358f885007168972
